Unparsing skips arguments with values equal to the argument type's default value

Using CommandLineParser 2.9.1, the unparsing behaviour with `SkipDefault = true` seems to be incorrect. Here is a test app to reproduce the behaviour.
```
using CommandLine;

namespace UnparsingTest
{
public class Program
{
public class TestArgs
{
[Option('c', Required = true)]
public int CompulsoryWithoutDefault { get; set; }

[Option('d', Required = true, Default = 1)]
public int CompulsoryWithDefault { get; set; } = 1;

[Option('o', Required = false, Default = 1)]
public int Optional { get; set; } = 1;
}

static void Main()
{
Parser parser = new Parser();

for (int i = 0; i <= 2; ++i)
{
TestArgs args = new TestArgs()
{
CompulsoryWithoutDefault = i,
CompulsoryWithDefault = i,
Optional = i,
};

string commandLine = parser.FormatCommandLine(args, settings => settings.SkipDefault = true);
System.Console.WriteLine($"CompulsoryWithDefault: {args.CompulsoryWithDefault}, CompulsoryWithoutDefault: {args.CompulsoryWithoutDefault}, Optional: {args.Optional}, Command line: \"{commandLine}\"");
}
}
}
}
```

There are 2 compulsory arguments, one with a `Default` value and one without a `Default` value. `CompulsoryWithDefault` and `Optional` have their `Default` values set to any value that is not `default(int)`.

The test app's console output shows the unparsed command line for the 3 sets of arguments
```
CompulsoryWithDefault: 0, CompulsoryWithoutDefault: 0, Optional: 0, Command line: ""
CompulsoryWithDefault: 1, CompulsoryWithoutDefault: 1, Optional: 1, Command line: "-c 1"
CompulsoryWithDefault: 2, CompulsoryWithoutDefault: 2, Optional: 2, Command line: "-c 2 -d 2 -o 2"
```

The unparsed command line for the first case will fail round trip parsing because the compulsory `-c` and `-d` arguments are missing and `-o` is missing so `Optional` will have a value of 1 instead of 0. The unparsed command line for the second case will also fail round trip parsing because the compulsory `-d` argument is missing. Only the third case is giving correct output.

The bug appears to be that the unparser is skipping `Required = true` arguments, arguments with value equal to `Default = {value}` and arguments with value equal to `default(typeof(argument))`.

The correct behaviour would be to only skip arguments that do not have `Required = true` and whose value equals the value specified in `Default = {value}`.

The expected console output would be
```
CompulsoryWithDefault: 0, CompulsoryWithoutDefault: 0, Optional: 0, Command line: "-c 0 -d 0 -o 0"
CompulsoryWithDefault: 1, CompulsoryWithoutDefault: 1, Optional: 1, Command line: "-c 1 -d 1"
CompulsoryWithDefault: 2, CompulsoryWithoutDefault: 2, Optional: 2, Command line: "-c 2 -d 2 -o 2"
```
These command lines would then correctly round trip parse/unparse.

提供された Program および TestArgs アプリで問題を再現し、settings.SkipDefault = true の Parser.FormatCommandLine に焦点を当てます。値 0、1、2 に対する出力を期待されるコマンドラインと比較し、その後、それぞれの結果をラウンドトリップで元の値に再度パースできることを確認します。

索引モデルが issue の本文から書いたものです。